It is known to weld two coaxial ferromagnetic workpieces, which are axially separated by a narrow peripheral gap, with the aid of an arc moving along that gap under the control of a generally radial magnetic field traversing the gap substantially at right angles to the developing seam. Such a field can be conventionally generated by surrounding each workpiece with an electromagnetic winding energized with such polarity that the resulting axially oriented lines of magnetic force buck each other and are effectively deflected onto a radial path forming part of a respective magnetic loop. These windings can be disposed at some distance from the gap, and thus from the region of the weld seam, but the introduction of the workpieces into the windings and especially their withdrawal after welding may create problems; moreover, the workpieces must also be retained in their aligned position by suitable holders whose emplacement may be impeded by the presence of these windings.
It has also been proposed (see, for example, German published application No. 28 15 751) to establish the desired magnetic field with the aid of two or more electromagnets disposed at angularly spaced locations in the plane of the inter-workpiece gap. These electromagnets have pole pieces whose arcuate faces are centered on the workpiece axis and closely approach the region of the weld seam with small angular separation from one another. Because of their close proximity to the location of the arc, these pole faces are susceptible to contamination by weld spatter; special holders must again be provided for engaging and centering the workpieces